FIRE RESEARCH AND RESULTS AT THE 

FOREST PRODUCTS LABORATORY 

Information Reviewed and Reaffirmed 

October 1959 

No. 1999 
- - - - 

FIRE RESEARCH AND RESULTS AT THE 

FOREST PRODUCTS LABORATORY1 

By 

T. R. TRUAX, Chief 

Division of Wood Preservation 

Forest Products Laboratory,2 Forest Service 

U. S., Department of Agriculture 

The performance of wood under fire conditions has been the subject of 

research and investigations for quite some time. Some of the early work 

in this field was done by the National Bureau of Standards, Underwriters, 

and Associated Factory Mutual Laboratories, and their affiliated fire 

insurance organizations, and certain municipalities, particularly 

New York City. It was aimed at establishing fire test procedures and 

defining perfomance standards for building construction. The results 

of these early tests influenced the use of wood in construction and the 

setting of insurance rates, 

Many important commercial products, including, of course, wood products, 

will burn and eventually be destroyed by fire. This fact, however does 

not prevent them from being used and enjoyed in abundance every day. Nor

does it mean that combustible products per se are less resistant to 

attack and damage by fire than many so-called incombustible materials. 

Steel beams, for example, will not burn, but they will soften and colLapse when exposed to high temperatures. The problem, then, is to learn 

to take advantage of the natural advantages of wood as, for example, its 

self-insulating properties, to improve structural details, and to develop 

greater resistance of wood to fire for applications where circumstances 

warrant. To accomplish these objectives research becomes important. It 

is my aim to survey briefly some of the research that has been done at 

the Forest Products Laboratory, to summarize the more important results, 

and to refer to some of the important work of others. 

A survey of past work indicates that research workers have sought to describe the performance of wood under fire conditions by defining three 

general characteristics, namely, (a) ignition, (b) fire spread, and 

1Presented at the meeting of the American Institute of Timber Construction 

in New Orleans, La., March 29, 1954. 

1954. 

Original .report dated September 

2Maintained at Madison, Wis., in cooperation with the University of 

Wisconsin. 

Report No. 1999 -1-
(c) resistance to penetration by fire, These characteristics arc to some 

extent interrelated. Ease of ignition, for example, is a factor in the 

spread of fire but the terms are by no means synonymous. In large-size 

timbers ignition is relatively Blow and fire spreads over them with considerable difficulty; in this case 'the two characteristics appear to be 

closely related. However, in finely divided wood materials, such as dry 

sawdust, ignition may be easy but fire spread slow. Again, decayed wood 

that has dried out ignites to glowing combustion relatively easily but 

spreads fire slowly. In these last two cases there is no close correlation between case of ignition and fire spread. "Likewise, the property of 

wood to resist penetration and destruction by fire does not appear to be 

directly related to ease of ignition or rate of fire spread. These three 

characteristics, therefore, have served as the approach to most fire research on wood and wood products, with either no attempt or frequent unsuccessful efforts at correlation. 

Fire research began at the Forest Products Laboratory about 1912, only a 

couple of years after the Laboratory was established. World War I interrupted the work, but it was revived in 1927 with some assistance and cooperation from the National Lumber Manufacturers Association. During the 

ensuing years investigations have continued on a relatively small scale, 

supplemented to some extent by financial support and cooperation from 

millwork, fiberboard, plywood, and other producers of forest products and 

from the Housing and Home Finance Agency. 

Ignition Studies 

The earliest fire studies at the Forest Products Laboratory involved ignition tests. In one series of tests Prince placed wood blocks 1-1/4 x 

1-1/4 x 4 inches in a heated air stream of approximately constant temperature in the presence of a pilot flame. The specimens were allowed to remain in the heated air stream until the gases ignited or for 40 minutes 

if ignition did not occur. The times for ignition were determined for 

several temperatures. By this technique tests were made on 9 species of 

wood untreated and l species treated with 9 different fire-retardant 

chemicals. The wood was tested both ovendry and air-dry. For air-dried 

blocks the temperatures at which the specimens could be heated for 

40 minutes without ignition varied from about 313° to 385° F. for untreated wood, and from about 290° to 500° F. for the one species of wood 

treated with fire retardants. In another series using the same test procedure on 34 untreated softwood and hardwood species, but igniting the 

specimens within 4 to 6-1/2 minutes, the temperatures varied from 608" to 

660° F. for the softwoods and from 595° to 740° F. for the hardwoods. 

The results of these tests, when plotted, show a fairly close relation 

between specific gravity and ignition temperature. In general low density 

species ignite at lower temperatures than high density species. This 

relationship was more consistent for hardwoods than for softwoods. 

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Later Van Kleeck studied the temperature at which exothermic heating 

develops in wood and from which heating logically progresses to the ignition point. Southern yellow pine untreated and impregnated with several 

retentions of fire retardants was used. A sample of small chips was exposed in an air stream of constant flow rate and uniform temperature increase, without a pilot flame. By progressively recording the time and 

the temperature in the air stream and within the sample, the point at 

which exothermic heating began was determined. For the untreated. wood 

this was between about 445° and 465° F. For zinc chloride treated wood 

exothermic heating began at a slightly lower temperature than for untreated wood; for ammonium phosphate treated wood it was somewhat, higher, 

although not determined accurately. 

A number of other research workers have used similar or modified methods 

in an attempt to determine the ignition .temperature of wood. For example, 

Brown of the Bureau of Standards, using a method similar to that employed 

by Van Kleeck, reported ignition temperatures of 378° to 428° F. for 3 

conifers and 2 hardwoods. Graf of the Oregon State College in more 

recent studies, using again a similar method, tested 20 species of wood. 

He reported ignition temperatures of 457° to 570° F. for 15 conifers and 

423° to 500° F. for 5 hardwoods. In both cases, the samples were small 

and of finely divided material, varying from shavings to match size. 

It is thus obvious that available data on the ignition temperatures of 

wood vary greatly. This is to be expected because of the large number of 

variables entering into such determinations. The species of wood, its 

size, form, and moisture content, the period of heating, air supply, the 

presence or absence of a pilot flame, and other factors affect the 

temperature at which wood will support flame or glowing combustion. Ignition temperature data, while informative, are not accurate criteria for 

judging the performance of wood as it is commonly used under fire conditions. 

Ignition temperature determinations have also been used by research 

workers in the hope of finding treatments that would substantially raise 

the ignition temperature of wood and thereby provide a method Of increasing its fire resistance. So far this approach has not accomplished significant results. Nor has it been possible to explain the properties of 

fire-retardant treated wood on the basis of ignition data. 

Fire Spread 

Up to about 1930 comparatively little attention had been given to determining fire spread on wood and wood structures by research workers. In 

the middle 1920's Dunlap conceived and developed preliminary plans for a 

fire tube as a means of comparing the rate of burning or fire spread over 

different species of wood, both untreated and treated with fire-retardant 

materials. After considerable further development and standardization, 

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the fire tube was used extensively at the Forest Products Laboratory. It 

was used for comparing the effectiveness of different chemical treatments 

for wood and the effect of various coatings on fire spread. The fire 

tube was later adopted as a fire-test method by the ASTM under designation E 69-70. 

The crib test, developed previously by the City of New York as one of 

three small-scale tests for determining the effectiveness of fire-retardant treatments, is a type of fire-spread test. It, too, was adopted by 

the ASTM as standard E 160-50. 

been devised. and used In measuring flame spread over wood and wood prodSeveral other small-scale tests have since 

ucts. 

Bureau of Standards, has been used for some time in evaluating fire-spread 

Of these, Federal Specification SS-A-118a test, developed by the 

characteristics of acoustical materials. The Schlyter test, developed 

in, Sweden, was modified somewhat and has been used considerably at the 

Forest Products Laboratory. The inclined panel test, adapted by the 

Forest Products Laboratory from British Standard No. 476-1932, is used. by 

the Insulation Board Institute and specified in Commercial Standard 

CS 42-49. 

In a study conducted at the Forest Products Laboratory in cooperation 

with the Insulation Board Institute, several of the above small-scale 

tests were used to evaluate the flame-spread resistance of fiber insulating boards. The material tested included 20 fiberboards both with and 

without fire-retardant coatings. Results of this study are reported in 

the Forest Products Laboratory Report No. 1756. 

The tunnel test developed by the Underwriters Laboratories followed the 

issuance of various reports giving the results from smaller scale firespread tests. It is a well known method and has been adopted as tentative standard E 84-50T by the ASTM. The Underwriters Laboratories, Inc., 

describe the tunnel test in Research Bulletin No. 32 entitled "Fire 

Hazard Classification of Building Materials." They have made extensive 

tests on lumber, plywood, wallboard, and other materials. However, (because of the size and complexity of the Underwriters Laboratories' equipment) attempts are underway3 at the Forest Products Laboratory in cooperation with ASTM Committee E-5 to develop a smaller scale piece of equipment that will simulate the performance of the larger tunnel test and be 

more readily adaptable to commercial and other testing laboratories. 

Fire-spread tests have also been made at the Forest Products Laboratory 

on structural assemblies containing combustible materials and contents. 

One of the earlier investigations on assemblies consisted of a series of 

tests to determine the probable ignition from incendiary bombs in the 

3The Forest Products Laboratory 8-foot tunnel-furnace apparatus has now 

been successfully developed (1959). Tests of materials in this furnace 

indicate good correlation and reproducibility for the method. The 

method and results are reported in Forest Products Laboratory Reports

Nos. 20 7 (1957) and 2140 (1959) and in ASTM Bulletin No. 230 (1958). The method is being considered or adoption as an ASTM Standard 

Report No. 1999 -4-
attics of buildings. A structural assembly representing the corner of an 

attic was used. Incendiary bombs of different types and sizes were 

placed on the attic floor and ignited, and various fire-retardant coatings 

were used in an attempt to reduce or prevent fire in the adjacent wood 

members. This method of test and results obtained were described by 

McNaughton in an article in the Quarterly of the National Fire Protection 

Association of January 1943. 

A second form of fire-spread tests used to a considerable extent at the 

Forest Products Laboratory is the semi-burn-out. For this purpose a 

specially constructed. fire-test house is employed. The walls and adjacent ceiling are covered with the material under test. A pilot fire is 

placed on the floor at the base of the wall. as an ignition source and 

allowed to burn out. The extent and velocity of fire spread over the 

wall and ceiling is recorded. This form of test has been made both on 

the side walls and in the corners of the room. With the same size of ignition source, the corner test is the more severe of the two. A number 

of wall coverings, including wallboards, lumber, and plywood have been 

tested. In one series of tests the results from a semi.-burn-out were 

compared with various small-scale fire tests. These various flame-spread 

test methods are described in Forest Products Laboratory Report No. 1443. 

Complete burn-out tests have also been made in the special fire-test room. 

Report No. 1941 entitled "Experimental Dwelling-Room Fires" gives the results of six such burn-out tests. Wall and ceiling coverings of insulation board, gypsum wallboard, plaster, and interior Douglas-fir plywood 

were tested. The fire was started in simulated furniture in the center 

of the room and allowed to burn until the entire interior of the room was 

involved, The report describes the nature and location of fuel, effect 

of wall coverings, temperature buildup, and composition. of the flue gases. 

A number of interesting results were obtained. Among the more significant was the buildup of temperatures, unbearable to a human being, from 

the burning of the contents of the room before the walls were ignited and 

before carbon monoxide had been produced in toxic concentrations. 

A number of other fire-testing agencies have investigated the fire-spread 

characteristics of various materials including wood and wood products. 

Of these the Factory Mutual. Laboratories have used a semi-burn-out procedure similar to that described above but on a larger scale. Their 

method of test and procedure is described in their laboratory report 

No. 11760. 

The British have studied fire spread experimentally in two full-scale twostory houses,4 The fires were started at the wall and somewhat different 

conclusions were drawn on the effect of the combustibility of the wall 

boards than in our experiments in which the fires were started in the 

furniture. 

4Combustible Linings in Dwelling Houses. Fire Protection Association 

No. 16, London, March 1953. 

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Fire Resistance 

Research on the resistance of wood to penetration by fire has likewise 

been the subject of considerable research. The Germans and English are 

reported to have done testing on the fire resistance of structural mem­ bers before active work was undertaken in this country. However, early 

work here dating back some 50 years was done by Columbia University, the 

Underwriters Laboratories, and the Bureau of Standards, which served as 

a basis for establishing the conditions under which building structures 

and materials are tested for fire resistance, The present time-temperature curve, which is part of ASTM Standard E 119-50 for fire tests of 

building construction and materials, was prepared in 1918. It has since 

come into use by practically all fire-testing agencies. 

Fire-test .facilities at the Forest Products Laboratory for determining 

the fire resistance of wood and wood-base materials were provided 

shortly after the new building was erected in 1932. These facilities 

have since been used for testing resistance of many materials and constructions, such as walls, doors, and floors. Most of such tests have 

been made without loads since the facilities are generally inadequate 

for applying loads to structural members under test. 

Walls 

An investigation was made of the fire resistance of "stressed covered" 

wall units used in prefabricated house design. It determined the degree 

to which fire resistance was affected by construction details, such as 

kinds of plywood, width of studs, and type of insulation. The program

involved tests on 2- x 2- and 4- x 8-foot sections at the Forest Products 

Laboratory (fig. 1) and 8- x 16-foot sections at the National Bureau of 

Standards. Report No. 1257 covers the results of tests made at the Forest 

Products Laboratory. A second series of fire-resistance tests of walls 

involved 23 sections, made in cooperation with the Insulation Board Institute. All structures were 4 x 8 feet made of 2- x 4-inch framing members with the interior wall covering of plaster and the exterior covering 

of redcedar siding. The sheathing and lath varied as to materials and 

thickness. Both interior and exterior wall coverings were exposed to the 

furnace fire. Other fire resistance tests on walls have involved paper 

core panels of sandwich type construction, 

Doors 

Tests of door assemblies have been made on the large furnace and of door 

sections on a small furnace. Most of this work has been done in coopera­ tion with the National Woodwork Manufacturers Association and individual 

manufacturers. 

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Report R1239 entitled "Fire Tests on Wood Doors" covers the results of 

full-scale door tests with two designs of solid core doors and on hollow 

core doors with and without insulation. Untreated and fire-retardant 

treated wood was used. A second series of tests was made on door sections 19 x 19 inches to determine the comparative fire resistance of 8 

types of flush door construction. All doors had solid core of wood or 

incombustible material. Treated and untreated wood was included in the 

test. Still other tests have been made on doors and sections of doors to 

determine the best types of fastenings so as to avoid early failures that 

normally occur in connection with hinges and locks. 

Ceilings 

A study was made of the temperature buildup between ceiling and joists 

under standard time-temperature conditions. The sections involved 

plaster and metal lath attached to standard wood joists with and without 

floors above the ceiling and with and without insulation between the 

studs. The primary object of the study was to determine if ASTM E119 

standard requirement limiting temperature rise to 250° F. between ceiling 

and wood joists is realistic. Observations were made on the time when 

glowing or flaming of the wood joists occurred and these observations 

were correlated with the temperature rise at the contact surfaces between 

joists and ceiling. All thermocouples placed between the ceiling and 

joists showed 250° F. temperature rise in from 8.5 to 10.6 minutes but in 

all cases the times when glowing and flaming were first observed were 

much longer. In some cases flaming or glowing was not observed until 

after an hour of exposure with a temperature rise between ceiling and 

joists of well over 1000° F. 

Heavy Timbers 

Fire tests on heavy timber construction have generally been of the resistance type. The National Bureau of Standards has made tests on columns 

and wood-framed structural elements and assigned fire-resistance ratings 

to them in report "BMS92 - Building Materials and Structures." The Underwriters' Laboratories reported on "Fire Exposure Tests on Loaded Timber 

Columns" in Research Bulletin No. 13. The Underwriters' Laboratories 

reported that timber columns, when given adequate end protection sustained the fire and load tests for 78 to 112 minutes before failure. 

The Factory Mutual Laboratories have also made extensive investigations 

of the fire resistance of heavy timber construe-Lion. 

Work at the Forest Products Laboratory on the fire resistance of heavy 

timbers has been limited. However, in connection with the construction 

of the laminated arch building at the Forest Products Laboratory, some 

fire-resistance tests were made on glue-laminated wood members. A section 

from one of the arches, consisting of 25 laminae of nominal 1-inch southern yellow pine glued with casein glue, was placed against a furnace with 

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one side exposed to a gas flame under standard temperature conditions. 

The edges of the glue lines were exposed to the fire. After 30 minutes' 

exposure the section was examined for depth and character of char. It 

was observed that the glue lines were separated to the depth of the char 

and that the depth of char at the glue lines was slightly deeper than in 

the wood laminations. Shear tests were also made on the glue lines of 

the uncharred part of the member. There Were indications that the shear 

strength immediately adjacent -to the charred area was reduced slightly, 

of the order of 7 to 10 percent, in comparison with the part of the member not exposed to the fire. 

A second series of tests was made on matched solid and laminated speci­ mens, also glued with casein glue. In these tests where the laminated 

beam was exposed so that the face of the laminated member was adjacent 

to the fire, the outer lamination became loose when the char reached the 

glue line. However, other tests have shown that glue bonds, made with 

phenolic, resorcinol, and melamine adhesives, hold the charred wood in 

place and the performance is similar to that of solid wood. 

To determine the rate at which wood chars when exposed to fire, another 

test was made that yielded some interesting information (figs. 2, 3, and 

4). The test was on a 4- x 8-foot Douglas-fir panel 7.56 inches thick, 

exposed on one side to approximately the standard time-temperature conditions. The panel was constructed by nailing together nominal 2 x 8 

inch planks 4 feet long. Eighteen thermocouples were distributed within 

the panel at depths of 1/1.6, 1, 2, 3, 4-l/2, and 6-1/2 inches from the 

face of the panel exposed to the fire. The wood. was at an average moisture content of 11.4 percent before test. 

The temperature in the furnace was measured by 6 thermocouples placed 6 

inches from the face of the panel. The fire was allowed to burn for 2 

hours and 20 minutes. The result of measurements on the panel after 

burning indicated that 0.48 inch of the original thickness had been lost 

by destruction of wood substance and shrinkage; 3.11 inches were charcoal 

and 3.91 inches were uncharred wood. The average rate of penetration of 

the zone of char into the wood was 1.54 inches per hour. 

In summary, the following general conclusions, based on past research 

and results obtained, are indicated: 

1. Investigations of the ignition temperature of wood, while informative, 

have not yielded results that can be used for predicting the actual performance of wood under fire conditions, nor in evaluating the significance 

of fire-retardant treatments and coatings. 

2. Fire spread is the characteristic of wood that is most affected by 

fire-retardant treatments and coatings. Size, form, density, moisture 

content, and arrangement of members are other factors that have an 

important affect on the fire-spread characteristic of wood. 

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3. Resistance of wood to destruction by fire, such as rate of penetration 

of fire and maintenance of structural properties under standard timetemperature conditions, is affected by its physical properties and structural details. Resistance to charring is not greatly increased by fireretardant treatments, perhaps at most by some 10 to 20 percent. 

4. Good performance of wood construction under fire conditions is obtained by taking advantage of the self-insulating qualities of wood, by 

employing good structural details, and by using fire-retardant treatments and coatings where circumstances warrant. 

5. There is much yet to be learned about wood and wood products in relation to fire. In view of the importance of fire in the construction 

field the research effort put forth by the Forest Products Laboratory and 

the wood utilization industries in the past has been relatively small and 

inadequate and a sustained and enlarged effort seems fully justified in 

the future. 

Report No. 1999 -9- .3-14 

Figure 2.--Built-up Douglas-fir member set in furnace for the 

start of test. Laminations 1-5/8 x 7-1/2 inches x 4 feet 

nailed together to produce panel 8 x 4 feet x 7-1/2 inches. 

Thermocouples are in place to measure temperatures on un­ exposed face and at depths of 1/16, 1, 2, 3, 4-1/2, and 

6-1/2 inches from the face of panel exposed to fire. 

ZM 53088 F 
Figure 3.--After 2 hours and 20 minutes fire exposure 

under approximate standard time-temperature conditions, 

the panel shown in Figure 1 was removed from furnace and 

cut through from top to bottom at its center, showing 

depth of charring and of uncharred wood. 

ZM 53071 F